Since at least 1882, as shown in U.S. Pat. No. 257,580 issued to Ditzler, parabolic trough shaped reflectors have been utilized to track the diurnal movements of the sun to focus the image of the sun upon a collection tube set at the focal point along the axis of focus for such a reflector. Since this early date, many attempts have been made to provide parabolic trough shaped concentrators for solar energy and all have met with a greater or lesser degree of success. Of course, one of the primary objects of the early art, which is continued forward to the present day, is the requirement that the collector be as faithfully parabolic as is possible or obtainable since slight inaccuracies of curvature will move the location of the focal point or axis off line, thus greatly reducing or eliminating the efficacy of such collectors. A second problem inherent in the prior art is the structural stability of the parabolic surface. Ideally, it would be desirable that the parabolic surface be formed or hewn from a solid block of material so that temperature variations, etc., would be uniformly compensated for and the line of focus for the trough shaped collector will remain unchanged.
In practice, the prior art has utilized structural frameworks with necessary stiffeners and arcuate ribs instead of the massive, expensive and unwieldy solid blocks of material. For example, the Podolny patent. U.S. Pat. No. 3,070,703, shows a solar power plant in which parabolic trough shaped collectors utilize a structural frame and stiffeners to support parabolically curved reflecting surface. U.S. Pat. No. 2,141,330 also shows a parabolic trough shaped collector formed of a framework intended to maintain the structural uniformity and integrity of the resultant concentrator.
Such structures require numerous, highly accurate parabolic arcuate support ribs which are expensive to make and which must be precise since the resulting form of the trough collector depends entirely upon the arcuate shape of the parabolic structural ribs. Some prior art devices have even resorted to adjustment screws or tensioning means to vary the parabolic surface for focusing reasons. A structure of this type is shown in British Pat. Specification No. 485,390 of May 19, 1938, in which an assembled structure of carefully machined ribs forms a concave mold to which a sheet of reflector material is permanently attached by fasteners. Much of the precision is lost because of eventual racking of the necessarily loose fit of bolt and rivets in drilled holes.
An attempt to alleviate the expense and difficulty of forming parabolic trough collectors utilizing a framework with numerous high precision parabolic ribs is shown in U.S. Pat. No. 3,959,056 in which a method of forming parabolic panels using honeycomb panels, foamed plastic, glass fibers, and impregnating resin is shown in which a continuous trough can be continuously molded to any length desired. Very expensive and complex equipment is required for such a forming technique and the resulting parabolic surface of the trough shaped collector may be subject to thermal distortion or other mechanical difficulties because of the nature of the materials utilized.
Another approach to reducing the cost of making reflectors has been to substantially sacrifice the degree of precision used in manufacture by using simplified molds on which the reflector's parabolic face is formed. Erdman, et al, U.S. Pat. No. 3,855,027, uses an air-inflated mold of flexible material on which a sheet of fiber reinforced polyester resin is fabricated by conventional spray lay-up technique. This sheet is then rendered suitably stiff and rigid by incorporating reinforcing ribbing in a second layer consisting of foamed polyurethane, followed by a third layer of spray lay-up fiber reinforced polyester. Even though the structure may retain reasonable rigidity in use, the vital precision required in the reflector surface can be no greater than that of the low pressure "balloon" on which it was formed (Column 4, lines 10-15) and this is given as a surface accuracy of 0.060 inches rms, Column 3, line 43. A similar technique is used by Chandler, U.S. Pat. No. 3,655,472, in which air pressure inflates an aluminum foil, which becomes the finished surface after layers of fiber reinforced resins are applied, again by spray lay-up. In view of the well known shrinkage characteristics of polymerization of resins during hardening, it is obvious that precision must be lost when this warping reaction is resisted only by an air inflated elastic substrate.
Nelson, U.S. Pat. No. 4,115,177, utilizes a rigid permanent mold, instead of air pressure, on which to fabricate a parabolic shaped sheet made by applying several layers of spray lay-up fiber reinforced thermosetting plastic, and finally vacuum depositing aluminum to form the reflector surface.
Some approaches have been made to improve the precision, yet retain the use of low cost molding, by substituting foamed polyurethane or the like for the air-inflated membrane as the form. Taplin, in U.S. Pat. No. 3,264,392 uses such a form, but in a male-female die forming technique. Schlager, U.S. Pat. No. 3,654,012, applies a heat softened sheet of plastic over a preformed mold of foamed polystyrene, actually melting the surface of the foamed plastic to create a bond with the sheet. Obviously these methods preclude any degree of precision, first because it is not possible to precision form foamed plastic, and second because heating and melting of the formed plastic alters the original contours.
In light of the foregoing difficulties mentioned with regard to the prior art, it is an object of the present invention to provide an improved method of forming precision parabolic reflector surfaces of the trough type for use as solar energy concentrators or collectors.